The present invention is directed to capacitor discharge engine ignition systems for small two and four stroke engines used in chain saw and weed trimmer applications, for example. The invention is more specifically directed to automatic control of engine ignition timing to obtain spark advance between starting and normal operating speeds, to inhibit ignition at excess engine operating speed and/or to prevent engine ignition at less than a minimum operating speed.
The time and occurrence of engine ignition is of importance to startability, output power and emissions performance of engines, including small two and four stroke engines. Optimum engine timing varies, primarily as a function of engine speed and load. Secondary factors, such as emissions performance and fuel quality, also play a role in determining optimum spark timing. Mechanical and microprocessor-based electronic timing control systems have been proposed for large engine applications, such as automotive engines, but are not well suited to small engine applications because of cost and packaging factors. Specifically, it has been proposed to employ microprocessor-based ignition modules in small engine applications, in which desired spark advance and/or retard timing characteristics may be programmed into the microprocessor. However, cost factors associated with microprocessor-based modules are prohibitive in small engine applications. It is a general object of the present invention to provide a capacitor discharge engine ignition system that is particularly well suited for small engine applications, that eliminates kick-back during starting, that facilitates manual starting of the engine, that facilitates engine carburetor timing, that exhibits increased power, that is relatively inexpensive, and/or that is well adapted for use in small two stroke and four stroke engine applications.
A capacitor discharge engine ignition system in accordance with one aspect of the present invention includes an ignition coil having a primary winding and a secondary winding for coupling to an engine ignition spark plug. A first electronic switch has primary current conducting electrodes in circuit with an ignition charge storage capacitor and the primary winding of the ignition coil, and a control electrode responsive to trigger signals for operatively connecting the ignition charge storage capacitor to discharge through the primary winding of the ignition coil. A charge/trigger coil arrangement generates periodic signals in synchronism with operation of the engine. The charge coil generates a charge signal to charge the ignition charge storage capacitor, while the trigger coil generates a trigger signal for triggering discharge of the capacitor through the ignition coil. An electronic circuit for controlling timing of the trigger signals as a function of engine speed includes a second electronic switch in the form of an SCR having primary anode and cathode current conducting electrodes operatively connected to the control electrode of the first electronic switch, and a control gate electrode. An RC circuit, including a resistor and a capacitor, is operatively connected to the charge coil and the gate electrode of the SCR to prevent application of trigger signals to the control electrode of the first electronic switch during occurrence of the charge signal, and thereby control timing of application of the trigger signal to the control electrode of the first electronic switch.
In accordance with a second aspect of the present invention, which may be implemented separately from or more preferably in combination with the first aspect of the invention summarized above, a capacitor discharge engine ignition system includes the ignition coil, first electronic switch, ignition charge storage capacitor and charge/trigger coil arrangement as described above. A third electronic switch has primary current conducting electrodes connected between the trigger coil and the control electrode of the first electronic switch, and a control electrode operatively coupled to the engine charge storage capacitor, preferably through a zener diode. The third electronic switch prevents application of trigger signals to the control electrode of the first electronic switch until the voltage on the ignition charge storage capacitor exceeds the zener breakdown voltage of the zener diodexe2x80x94i.e., when engine speed exceeds a preselected level.